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# Earthquake Epicenters and Hypocenters Help

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By McGraw-Hill Professional
Updated on Sep 4, 2011

## Hypocenters Vs. Epicenters

The hypocenter of an earthquake is the point at which the earthquake slip starts. Since earthquakes are usually caused by tectonic activity, hypocenters are always located at some depth underground.

The hypocenter of an earthquake is the location beneath the Earth’s surface where a fault rupture begins.

If you have ever seen television coverage of an earthquake, you probably saw the epicenter pointed out on a map. An earthquake epicenter is the point on the Earth’s surface directly above the hypocenter. Since this is an easy way to map an earthquake’s location, large earthquakes are often named after local geography, such as cities, rivers, or towns closest to the epicenter like the 1906 San Francisco earthquake. The epicenter is often found by comparing the distance from several different measuring stations in a network. The main function of seismograph networks is to find earthquakes. Figure 12-3 shows how the focus of an earthquake radiates and compares to the epicenter.

Fig. 12-3. The focus of an earthquake radiates out in all directions.

Although it is possible to find the general location for an earthquake from the information of a single station, it is more precise to use three or more stations. Pinpointing the source of an earthquake helps in evaluating its damage and in relating an earthquake to a geologic site.

The epicenter of an earthquake is the location directly above the hypocenter on the surface of the Earth.

The hypocenter and epicenter may be far apart. A deep hypocenter oriented at an angle away from vertical would not be found along the fault line, but at some distance away. Figure 12-4 illustrates how the epicenter and hypocenter compare to each other for angled faults and blind faults.

Fig. 12-4. The epicenter is the surface point above the earthquake’s hypocenter.

## Seiche

Have you ever tried carrying a full bucket of water across a distance of 20–30 m? It’s tough. Unless you walk very slowly and carefully with little sideways or vertical movement, water sloshes all over the place. The same thing happens after an earthquake. Lakes, swimming pools, fish tanks, and other bodies of water slosh out of their banks in response to the wave affect.

The sloshing of water from a swimming pool or any body of water, after an earthquake is known as a seiche . It can go on for a few minutes or up to a few hours; long after the vibrating force has stopped.

To give you an idea of the tremendous force of earthquakes, in 1985, it was reported that the swimming pool at the University of Arizona in Tucson lost water from sloshing (seiche) as a result of the 8.1 magnitude, Michoacan, Mexico earthquake 2000km away.

Practice problems of this concept can be found at: Earthquake Practice Test

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